REFRACTORIES  MANUFACTURERS 
ASSOCIATION’S 

INDUSTRIAL  FELLOWSHIP  No.  1 


SPECIAL  REPORT 


A  STUDY  OF  THE  TUNNEL  KILN  AND  ITS 
APPLICATION  TO  THE  BURNING 
OF  REFRACTORIES 


BY 

RAYMOND  M.  HOWE 


INDUSTRIAL  FELLOW 

MELLON  INSTITUTE  OF  INDUSTRIAL  RESEARCH 
UNIVERSITY  OF  PITTSBURGH 


PITTSBURGH,  PENNSYLVANIA 
SEPTEMBER  19, 

19  17 


PRESS  OF  REED  a  WITTING  CO. 
PITTSBURGH 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/studyoftunnelkilOOhowe 


REFRACTORIES  MANUFACTURERS 

& 

ASSOCIATION’S 

• 

INDUSTRIAL  FELLOWSHIP  No.  1 

SPECIAL  REPORT 

A  STUDY  OF  THE  TUNNEL  KILN  AND  ITS 
APPLICATION  TO  THE  BURNING 
OF  REFRACTORIES 

THERE  is  in  all  probability  no  more  vital  a  problem  in  the 
manufacture  of  refractories  than  that  of  burning.  The 
ware  must  be  burned  uniformly,  thoroughly,  and  as 
quickly  as  possible,  in  order  to  secure  the  maximum  capacity 
of  the  kilns.  At  the  present  time  many  plants  are  either  forced 
to  use  a  poor  quality  of  coal,  pay  exhorbitant  prices,  or,  in  some 
cases,  to  work  their  own  mines.  Under  such  conditions,  which 
probably  will  not  be  overcome  for  some  time,  the  manufacturer 
naturally  wishes  more  than  ever  before  to  obtain  the  maximum 
output  from  the  coal  used. 

Exact  figures  are  not  available  which  give  the  percentage  of 
heat  used  in  maturing  the  ware,  that  which  passes  out  of  the 
stack,  the  radiation  loss,  etc.  It  is  known,  however,  that  only 
five  to  twenty  per  cent,  of  the  heat  liberated  in  intermittent 
kilns  used  for  other  purposes  actually  enters  the  ware.  From 
thirty-three  to  seventy-one  per  cent,  is  lost  by  radiation  and  in 
the  kiln,  while  another  twenty  to  fifty-five  per  cent,  is  carried 
out  by  the  hot  stack  gases. 

It  is  the  latter  loss  that  is  most  directly  overcome  by  con¬ 
tinuous  kilns.  Continuous  kilns  of  the  tunnel  type  also  have 
a  Very  low  loss  due  to  radiation  as  well  as  a  small  loss  to  the  kiln. 
The  hot  waste  gases  are  used  to  “water-smoke”  the  incoming 
ware,  heat  it  to  the  temperature  at  which  the  combined  water 
is  driven  off,  and  even  to  start  the  various  chemical  reactions 
incident  to  the  development  of  a  finished  refractory. 


799751 


A  kiln  capable  of  doing  this  has  necessarily  attracted  much 
attention.  The  General  Electric  Company  has  investigated 
the  kiln  abroad  and  in  this  country.  The  Bureau  of  Standards 
has  made  a  heat  balance  of  the  tunnel  kiln  at  Keasbey.  Many 
other  companies  have  applied  the  kiln  to  the  production  of 
various  kinds  of  ceramic  and  metallurgical  ware. 

Mr.  L.  E.  Barringer  has  ably  described  the  kiln*  as  follows 
(Trans.  Am.  Cer.  Soc.,  18,  111-116): 

The  kiln  is  simply  a  straight  tunnel,  197  feet  in  length,  with 
a  fixed  firing  zone  somewhat  nearer  the  discharge  end  than  the 
charging  end.  A  track  extends  through  the  kiln  and  upon  this 
the  cars  are  moved,  being  gradually  brought  up  to  the  firing 
zone,  given  the  necessary  period  of  exposure  to  the  maximum 
temperature  and  then  passed  to  the  cooling  end  of  the  kiln. 

By  referring  to  Figure  A  the  general  operation  of  the  kiln 
may  be  explained  as  follows: 

At  the  charging  end  of  the  kiln  “A”,  loaded  cars  are  periodic¬ 
ally  “fed”  into  the  kiln,  being  first  placed  in  a  vestibule.  This 
vestibule  is  necessary  to  prevent  disturbance  of  the  draft  while 
the  inner  door  of  the  kiln  is  opened  to  receive  the  loaded  cars. 
After  the  entering  car  is  in  the  vestibule  and  the  outer  door 
closed,  the  inner  door  of  the  kiln  is  opened  and  the  car  moved 
into  the  kiln  proper,  pushing  all  preceding  cars  before  it. 

Movement  of  the  cars  is  accomplished  by  a  hand-windlass 
operating  an  endless  chain  just  below  the  cars  at  the  charging  end 
of  the  kiln.  This  chain  is  provided  with  hook  lugs  which  engage 
the  axles  of  the  car  and  move  it  forward  as  the  windlass  is 
operated.  To  facilitate  the  pushing  of  the  entire  train  of  cars, 
the  track  through  the  kiln  floor  has  a  slight  drop  from  the  charg¬ 
ing  end  to  the  discharging  end. 

As  each  additional  car  of  ware  is  placed  in  the  kiln,  the 
entire  “train”  of  cars  in  the  kiln  progresses  car-length  by  car- 
length,  gradually  passing  through  zone  B,  which  for  some  wares 
might  be  called  the  “water-smoking”  zone,  into  zone  C  and 
thence  into  the  firing  zone  D,  where  the  highest  heat  is  imparted 
to  the  ware.  After  the  car  of  ware  has  remained  in  the  firing 
zone  a  sufficient  length  of  time  to  bring  about  the  desired  shrink- 

*Tunnel  kiln  of  the  Didier-March  type. 


4 


age  in  the  ware,  as  indicated  by  pyrometric  cones  or  other  means, 
it  passes  into  the  cooling  zone  E,  where  the  material  is  cooled 
slowly  and  uniformly.  The  car  leaves  the  kiln  at  F  in  such  con¬ 
dition  that  the  material  can  at  once  be  taken  to  storage  and  un¬ 
loaded,  or,  if  somewhat  too  hot,  can  at  least  be  sidetracked 
without  interference  with  the  operation  of  the  kiln.  This  is  an 
advantage  in  favor  of  the  tunnel  kiln,  since  in  periodic  kilns  too 
hot  to  unload,  the  kiln  space  must  be  occupied  until  the  jvare 
can  be  taken  out. 

At  a  point  “G”  between  the  charging  and  discharging  ends 
of  the  kiln  the  four  grates  for  direct  firing  are  located,  two  on 
either  side,  and  form  the  base  of  the  combustion  chamber.  This 
combustion  chamber  is  separated  from  the  main  tunnel  by  an 
inner  wall,  and  the  products  of  combustion  pass  through  the 
tuyeres  H  and  enter  the  main  tunnel  where  they  become  diffused 
and  circulate  around  the  ware,  successively  passing  through  and 
into  the  zones  C,  D,  and  B.  At  the  point  J  the  waste  products  of 
combustion  and  steam  pass  out  of  the  kiln  into  the  stack. 

Air  for  combustion  enters  the  kiln  through  ports  in  the  walls 
at  the  discharge  end.  The  air  passes  through  “serpentine’’  flues 
and  becomes  preheated  by  contact  with  the  cooling  ware.  As 
the  ware  cools,  the  heat  units  are  given  up  to  the  in-coming  cold 
air,  and  this  preheated  air  passes  on  until  when  nearly  to  the 
firing  zone  it  is  divided,  part  of  the  air  passing  under  the  grate 
bars  to  be  used  for  combustion  (primary  air)  and  part  of  the  air 
being  deflected  into  the  kiln  chamber  to  complete  combustion 
of  the  fuel  gases  (secondary  air).  The  heated  combustion  gases 
pass  through  the  tunnel  towards  the  charging  end  and  heat  up 
the  in-coming  ware,  leaving  the  kiln  through  the  stack  flue  at 
a  point  about  ten  feet  from  the  charging  end  of  the  kiln,  as  in¬ 
dicated  by  J. 

The  kiln  has  a  capacity  of  36  cars,  and  one  car  is  entered  and 
one  removed  from  the  kiln  every  two  hours.  A  car  thus  requires 
seventy-two  hours  to  pass  through  the  tunnel. 

An  important  feature  of  the  cars  is  the  “sand  seal’’  to  pre¬ 
vent  the  heat  of  the  kiln  from  finding  its  way  beneath  the  cars 
where  it  would  soon  damage  the  wheels  and  axles,  to  say  nothing 
of  disturbing  the  draft.  This  consists  of  sheet-iron  aprons  or 


6 


shields  which  move  through  troughs  of  sand  on  either  side  of  the 
tunnel  wall. 

The  tunnel  is  provided  with  a  “subway”  which  extends  the 
entire  length  of  the  kiln  beneath  the  cars  and  from  which  the 
heat  is  excluded  by  the  “sand  seal.” 

Youghiogheny  coal  is  used  at  Keasbey  for  firing,  and  the 
four  fire  boxes  are  fired  one  at  a  time  every  fifteen  minutes,  so  as 
to  maintain  as  closely  as  possible  the  same  conditions  of  oxida¬ 
tion  and  reduction,  as  well  as  temperature.  About  three  tons 
of  coal  are  used  daily. 

The  kiln  at  Schenectady  is  used  in  the  firing  of  porcelain 
insulators  to  a  temperature  of  1 300- 1 400°C.,  this  class  of  ware 
of  course  all  being  fired  in  saggers.  The  kilns  at  Keasbey  are 
used  in  the  firing  of  fire-brick  and  refractories  at  practically  the 
same  temperature,  and  in  both  cases  the  uniformity  of  distribu¬ 
tion  of  temperature  is  well  within  one  cone. 

As  an  indication  of  the  efficiency  of  the  kiln  it  may  be  noted 
that  the  temperature  of  the  escaping  gases  is  very  low,  averaging 
200-220°C.  at  Schenectady  and  1  50-200°C.  at  the  damper  in  the 
Keasbey  kilns. 

In  the  kilns  at  Keasbey  650  pounds  of  coal  are  used  to  burn 
1000  standard  fire-brick.  The  continuous  kiln  turns  out  about 
as  much  ware  as  three  and  one-half  fourteen-foot  periodic  kilns. 
The  cost  of  coal  is  about  one-half  that  for  the  periodic  kilns  of 
the  same  capacity.  Labor  cost  has  been  found  to  be  10  per  cent, 
less  for  loading  and  unloading,  and  the  labor  cost  of  firing  is  the 
same  as  in  periodic  kilns. 

Such  are  the  results  of  Barringer’s  investigation  on  this  type 
of  kiln.  Since  the  publication  of  his  article,  however,  several 
questions  have  arisen.  What  is  the  temperature  attained?  How 
uniform  is  this  temperature  in  various  parts  of  the  kiln?  Are  the 
refractories  thoroughly  burned  to  final  shrinkage?  Do  any 
strains  result  from  the  rapid  application  of  heat?  What  is  the 
breakage,  the  saving  in  fuel,  the  cost  of  setting,  the  cost  of  un¬ 
loading  and  the  cost  of  firing?  Are  there  many  ‘Turnovers?” 
Finally,  is  the  kiln  as  efficient  as  possible? 


7 


In  order  to  answer  these  questions  a  study  of  the  tunnel 
kiln  at  Keasbey  was  undertaken*.  This  kiln  was  operating  at 
the  proper  temperature,  and  was  turning  out  refractory  ware. 
The  results  obtained  should  apply  to  any  well  controlled  tunnel 
kiln. 

Fire-brick,  both  burned  and  green,  were  secured  which  were 
typical  for  the  various  parts  of  the  countryf .  The  green  brick 
were  measured  with  a  micrometer,  labeled,  and  placed  in  eleven 
cars.  These  cars  were  introduced  at  various  intervals  so  that 
the  first  test  was  extended  over  a  period  of  six  days. 

The  cones  on  top  of  the  car  were  often  vesicular  in  structure, 
being  made  from  natural  clay,  and  so  these  were  compared  with 
Orton  cones.  Cones  were  placed  with  all  of  the  brick,  and  forty- 
one  sets  were  scattered  so  as  to  measure  all  possible  temperature 
conditions  in  the  more  sheltered  interior  of  the  car. 

After  the  samples  were  burned  they  were  again  measured 
and  then  tested  for  porosity,  apparent  specific  gravity,  crushing 
strength,  and  traverse  strength.  This  same  data  were  secured 
from  the  stock  brick  on  hand,  it  being  necessary,  however,  to 
calculate  the  fire  shrinkage  of  these  from  the  average  dimensions 
of  the  green  brick. 

The  crushing  strength  was  calculated  in  pounds  per  square 
inch,  the  tests  being  made  on  the  sides  of  the  brick.  These  tests 
were  made  in  order  to  determine  whether  the  bond  had  been  as 
thoroughly  developed  in  the  tunnel  kiln  as  in  the  periodic  kilns. 

The  traverse  strength  tests  were  made  between  two  parallel 
fulcrums,  six  inches  apart.  It  is  given  in  pounds  per  brick.  This 
test  was  designed  as  a  means  of  discovering  any  cracks  which 
might  be  developed. 


*The  writer  wishes  to  acknowledge  his  indebtedness  to  the  Didier-March 
Company,  and  especially  to  Mr.  G.  A.  Balz,  for  hearty  co-operation  in  carry¬ 
ing  out  this  investigation. 

fSome  of  these  have  been  delayed  in  transit  and  have  not  been  tested  at 
this  time.  These  will  be  available  in  a  short  time. 


8 


Results  of  the  Tests 


The  temperature  variations  found  are  shown  diagramatically. 
Some  of  the  cars  were  burned  to  cone  13  and  some  were  burned 
to  cone  14.  In  order  to  simplify  the  tabulation  of  results  each 
car  was  considered  as  having  been  burned  to  cone  1 4.  The  neces¬ 
sary  corrections  were  made.  The  results  are  given  below.  At  no 
time  did  the  cars  receive  a  burn  of  less  than  cone  13.  In  order, 
then,  to  calculate  the  minimum  temperatures,  simply  subtract 
one  cone  from  the  temperatures  given  in  the  diagrams. 

The  physical  tests  have  likewise  been  tabulated  in  as  simple 
a  form  as  possible.  From  12  to  24  measurements  of  the  dimen¬ 
sions  of  the  brick  were  made  as  the  foundation  of  the  fire  shrink¬ 
age  calculations.  The  porosity  tests  were  made  on  four  samples 
of  each  brand.  Crushing  strength  tests  were  made  on  from  4  to 
12  samples  of  each  brand,  more  being  made  on  the  bricks  which 
showed  a  difference  in  quality  due  to  the  two  methods  of  burning. 
The  average  results  are  given  in  the  following  table: 

Cone  Equivalents  in  Temperature  Degrees 

Cone  Numbers  Centigrade  Degrees  Fahrenheit  Degrees 


10 

1330 

2426 

11 

1350 

2462 

12 

1370 

2498 

13 

1390 

2534 

14 

1410 

2570 

Average  Physical  Tests 


Sample 

Percentage 

Porosity 

Per  Cent. 

Fire  Shrinkage 

Crushing  Strength 
in  Lbs.  per  Sq.  In. 

T  raverse  Strength 
in  Lbs.  per  Brick 

Stock 

Brick 

Tunnel 
Kiln  Brick 

Stock 

Brick 

Tunnel 
Kiln  Brick 

Stock 

Brick 

Tunnel 
Kiln  Brick 

Stock 

Brick 

Tunnel 
Kiln  Brick 

I 

28.2 

27.7 

.70 

1.53 

1890 

1833 

1100 

1140 

II 

24.1 

22.1 

3.23 

3.68 

2029 

2096 

1185 

1670 

III 

21  1 

20.6 

3.05 

2.24 

1750 

1919 

1550 

1450 

IV 

26.3 

25.6 

3.89 

4.75 

1772 

1621 

1450 

1440 

V 

17.7 

17.8 

2. 18 

2.30 

2684 

1117 

1570 

1145 

VI 

23.1 

20.8 

2.86 

4.57 

2044 

1818 

1275 

1350 

VII 

21.5 

23.1 

1  .61 

1  .47 

1337 

819 

1220 

1250 

VIII 

17.4 

20.3 

4.67 

3.78 

5846 

5204 

3250 

3050 

9 


LAYER  I— (BOTTOM  OF  CAR) 


10 


LAYER  IV 


11 


LAYER  VII 


13- 

13- 

13 

13 

13 

LAYER  IX 

12- 

12  + 

12  + 

12 


LAYER  X 


LAYER  XI 


13 


LAYER  XII 


LAYER  XIII— (TOP  OF  CAR) 


14 


Discussion  of  Cone  Tests 


The  variation  of  cones  which  was  found  is  much  greater 
than  that  given  by  Barringer.  This  may  be  easily  explained. 
At  the  time  of  his  investigation  a  ware  was  being  burned  which 
required  a  uniform  temperature,  which  was  obtained  by  careful 
firing  and  the  use  of  new  cars.  The  present  investigation  was 
conducted  under  more  adverse  conditions.  The  cars  were  in 
need  of  repair  and  allowed  cold  air  to  seep  up  from  the  “subway” 
below.  The  kiln  was  not  fired  as  carefully.  Furthermore  it  had 
been  running  at  maximum  capacity  for  four  years.  Since  the 
ware  was  mature  at  cone  10  no  attempt  was  made  to  control  the 
burning  more  carefully,  this  variation  being  considered  per¬ 
missible. 


Discussion  of  Physical  Tests 

I.  These  brick  were  made  by  the  stiff  mud  process,  using 
an  auger  machine,  and  were  steam  repressed.  New  Jersey  fire¬ 
clay,  grog  and  gannister  were  used  in  the  batch.  The  ware 
burned  in  the  tunnel  kiln  showed  the  same  physical  properties 
as  the  ware  from  the  intermittent  kilns.  The  slight  difference 
observed  was  more  likely  due  to  experimental  error  rather  than 
conditions  of  burning. 

II.  These  brick  were  made  by  the  soft  mud  process  from 
13  per  cent,  bond  clay  and  83  per  cent,  flint  clay,  part  of  which 
had  been  calcined.  The  clays  were  from  the  Cambria  district. 
The  ware  coming  from  the  tunnel  kiln  showed  greater  shrinkage, 
greater  strength,  and  lower  porosity,  all  of  which  indicated  that 
the  tunnel  kiln  burn  was  more  thorough. 

III.  These  brick  were  much  the  same  as  those  of  type  B — 
being  higher  in  flint  clay.  The  ware  from  the  two  types  of  kilns 
was  essentially  the  same. 

IV.  These  brick  were  from  the  Southern  Ohio  district. 
The  two  burns  showed  practically  the  same  quality  of  ware. 

V.  These  brick  contained  twenty  per  cent,  bond  clay  and 
eighty  per  cent,  flint  clay,  part  of  which  was  one-half  an  inch  in 
diameter.  These  large  particles  of  flint  clay  often  protruded 
from  the  sides  of  the  firebrick  as  they  came  from  the  tunnel  kiln. 


15 


A  great  many  surface  cracks  were  found  in  the  brick  coming 
from  the  tunnel  kiln.  The  brick  were  also  weaker  than  those 
from  the  regular  burn,  showing  that  the  surface  cracks  evidently 
extended  throughout  the  brick. 

VI.  These  brick  contained  ninety  per  cent,  flint  clay  and 
ten  per  cent,  bond  clay.  They  showed  that  they  had  received  a 
harder  burn  in  the  tunnel  kiln  than  in  the  intermittent  kiln. 

VII.  These  brick  were  from  the  Pennsylvania-Ohio  dis¬ 
trict  and  were  made  by  the  soft  mud  process,  twenty-five  per 
cent,  bond  clay  and  seventy-five  per  cent,  flint  clay.  The  brick 
were  discolored  and  cracked  as  they  came  from  the  tunnel  kiln, 
showing  a  large  amount  of  iron  both  on  the  surface  of  and 
throughout  the  brick.  The  ware  from  the  intermittent  kiln 
showed  some  traces  of  iron  but  it  was  not  discolored  as  was  the 
ware  from  the  tunnel  kiln. 

VIII.  These  brick  were  made  with  an  auger  machine  from 
the  same  batch  as  given  in  VII.  The  stock  brick  were  nearly 
white,  dense  and  very  strong.  The  ware  coming  from  the  tunnel 
kiln  was  spotted  and  slightly  weaker. 

General  Conclusions 

I.  Fire-brick  made  from  calcined  clay  grog  and  bond  clay 
may  be  successfully  fired  in  the  tunnel  kiln.  The  quality  of  the 
ware  coming  from  the  tunnel  kiln  is  of  a  quality  equal  to  that 
coming  from  the  periodic  kiln. 

II.  Fire-brick  made  from  flint  clay  and  bond  clay  may  be 
successfully  fired  in  the  tunnel  kiln.  The  flint  clay,  however, 
should  not  be  introduced  in  too  large  particles  if  the  ware  is  to 
be  burned  and  cooled  in  seventy-two  hours.  If  some  of  the 
flint  clay  has  been  previously  calcined  very  good  results  are 
secured. 

III.  When  the  bond  clay  becomes  dense  at  a  low  tempera¬ 
ture,  difficulties  are  liable  to  arise  when  such  a  clay  is  burned  in 
the  tunnel  kiln.  Bricks  of  types  seven  and  eight  show  this.  It 
is  believed  that  such  brick,  those  which  become  very  dense, 
should  be  fired  to  a  lower  temperature,  or  else  be  fired  in  a  longer 
tunnel  kiln.  Either  one  of  these  porcedures,  if  followed,  would 
result  in  the  lessening  of  the  severity  of  the  heat  treatment. 


16 


IV.  Iron,  if  present,  results  in  the  formation  of  dark  spots 
on  the  surface  of  the  brick  coming  from  the  tunnel  kiln.  If  an 
excess  of  air  is  present,  such  as  is  found  in  the  periodic  kiln,  it 
oxidizes  the  iron  and  its  color  is  less  pronounced.  This  excess 
of  air  lowers  the  efficiency  of  the  kiln.  As  a  result,  if  the  high 
efficiency  of  the  tunnel  kiln  is  to  be  retained,  such  iron  spots  are 
unavoidable. 

V.  Brick  were  also  sent  through  the  tunnel  kiln  which  were 
made  by  the  dry  press  method.  They  were  packed  in  such  a  way 
that  the  original  moisture  content  was  present  when  the  bricks 
were  introduced  into  the  kilns.  The  results  were  not  satisfactory 
for  two  reasons.  The  brick  are  ordinarily  burned  to  cone  10; 
here  they  were  burned  to  cone  14  and  in  a  very  short  time.  As 
a  result  cracks  were  developed.  Were  the  kiln  longer  the  brick 
would  not  have  been  subjected  to  such  a  strain  during  the  water¬ 
smoking  period.  It  is  believed  that  by  using  a  longer  kiln  that 
dry  pressed  bricks  can  be  set  directly  on  the  tunnel  kiln  car  and 
be  burned  successfully. 

VI.  By  using  a  kiln  of  suitable  length,  properly  proportion¬ 
ing  the  raw  materials,  properly  sizing  the  raw  materials,  and 
burning  to  a  proper  temperature  there  appears  to  be  no  reason 
why  all  refractory  fire-clay  brick  can  not  be  successfully  burned 
in  the  tunnel  kiln. 

Miscellaneous  Details 
1.  Setting 

The  brick  are  set  on  a  car  such  as  is  shown  in  Figure  B.  Fig¬ 
ures  C,  D,  E,  F,  G,  H,  and  I  indicate  the  manner  in  which  the 
brick  are  set.  Layers  six,  ten,  and  two  are  similar;  layers  three, 
seven,  and  eleven  are  alike;  layer  eight  is  like  layer  four  and 
layer  nine  is  the  same  as  layer  five.  The  upper  layers,  twelve 
and  thirteen,  conform  to  the  shape  of  the  roof  of  the  kiln. 

The  lower  layers  are  set  openly  in  order  to  secure  a  better 
draft  in  the  portion  of  the  car  which  is  naturally  coolest.  Each 
layer  is  so  placed  that  an  interlocking  effect  is  secured  which 
overcomes  any  tendency  for  the  load  to  be  dislodged  while  pass¬ 
ing  through  the  kiln. 

Four  men  were  able  to  set  seven  cars,  each  of  which  held  from 
920  to  950  brick,  in  105  minutes,  or  one  car  every  fifteen  minutes. 


17 


SCALE—  1"  =8" 


Front  View  of  Tunnel  Kiln  Car 


Fig.  B. 


18 


Fire  Clay  Top  b — Sand  Shields  c— Iron  Framework 


LAYER  ONE 


Front  of  Car 


SCALE—  1 '» =  10 


19 


LAYER  TWO 


20 


LAYER  THREE 


Fig.  E. 


21 


LAYER  FOUR 


22 


LAYER  FIVE 


23 


LAYER  TWELVE 


24 


TOP  LAYER  THIRTEEN 


Fig.  I. 


25 


In  ordinary  practice  they  work  from  six  to  seven  hours  a 
day  in  setting  the  cars  for  one  tunnel  kiln,  including  two  extra 
cars  per  day  for  the  Sunday  supply.  If  it  were  not  for  the  large 
amount  of  special  shapes  sent  through  with  the  fire-brick  still 
better  time  could  be  made. 

2.  Fuel  and  Firing 

Barringer  reported  the  use  of  650  pounds  of  coal  per  1000 
fire-brick.  At  the  time  of  this  investigation  748  pounds  were 
used  for  1000  fire-brick. 

Two  firemen  were  necessary  for  one  kiln,  one  in  the  day 
time,  who  also  oiled  the  cars,  and  one  night  fireman.  One  man 
hauled  the  ashes  from  two  kilns. 

These  same  firemen,  with  the  aid  of  other  men  from  the 
plant,  introduced  the  new  cars  into  the  kiln  and  drew  the  finished 
cars.  Four  or  five  men  were  required  for  this  operation  and  spent 
fifteen  minutes  every  two  hours  in  doing  it. 

3.  Unloading 

Two  men  unloaded  twelve  cars  of  brick  in  seven  hours.  The 
time  required  for  unloading  standard  brick  was  35,  30,  16,  18,  16, 
and  35  minutes  per  car  of  920-960  brick.  They  experienced  the 
same  difficulty  as  the  setters  in  working  with  a  variety  of  ware. 
Were  the  kiln  used  for  standard  brick  alone  the  two  men  could 
undoubtedly  accomplish  more. 

Two  other  men  took  the  brick  from  the  unloaders  and  placed 
them  in  the  yard,  on  the  boat,  or  in  the  car. 

4.  Breakage 

The  kiln  breakage  for  standard  fire-brick  was  extremely 
small.  2,  2,  5,  4,  1,  and  3  broken  brick  were  found  on  six  cars 
of  standard  brick,  or  about  0.3  per  cent.  When  mixed  cars  were 
burned,  as  was  generally  the  case,  the  breakage  was  naturally 
higher.  It  was  however  less  than  1 .0  per  cent.,  there  being  about 
fifty  broken  fire-brick  from  every  12,000  output. 

5.  Upkeep 

The  kiln  investigated  had  been  in  continuous  operation  for 
four  years.  Each  part  of  the  kiln  had  been  maintained  at  ap- 


26 


proximately  the  same  temperature  for  the  entire  period.  Be¬ 
cause  of  this  the  deterioration  in  a  kiln  due  to  repeated  heating 
and  cooling  is  avoided. 

The  cars  last  about  two  years,  by  which  time  the  tops  be¬ 
come  cracked  and  uneven.  One  man  can  repair  two  of  these 
cars  in  a  day. 

6.  Adaptability 

Large  pieces  of  ware  were  sent  through  the  tunnel  kiln 
without  experiencing  breakage.  Since  such  special  shapes  could 
be  secured  four  days  after  being  set,  the  kiln  was  used  largely  for 
the  burning  of  such  ware.  In  one  instance  a  gas  retort  was  burned 
successfully  in  this  manner,  although  it  is  not  the  usual  practice. 

Possible  Improvements 

The  kiln  is  very  easily  changed  to  suit  conditions,  and  offers 
a  marked  contrast  in  this  respect  to  the  periodic  kiln. 

Producer  gas,  natural  gas,  and  oil  have  been  successfully 
used  as  fuel.  Because  of  the  location  of  the  fire-boxes,  the  fuel 
may  be  easily  interchanged;  i.  e.,  natural  gas  may  be  used  in  the 
summer  and  producer  gas  in  the  winter.  If  the  value  of  one  fuel 
increased  and  another  does  not,  advantage  may  be  taken  (in 
fact,  it  has  already  been  done),  of  the  changing  market  con¬ 
ditions. 

Gravity  conveyors  for  handling  the  burned  bricks  can  also 
be  applied  to  this  type  of  kiln.  Since  the  brick  are  always  found 
at  the  same  place,  the  position  of  the  conveyor,  once  being  fixed, 
need  not  be  changed. 

The  tunnel  kiln  car  itself  might  be  moved  directly  to  the 
railroad  car,  or  into  the  yard  of  a  well  designed  plant. 

Professor  Harrop  has  found  that  a  large  amount  of  heat  is 
lost  by  radiation.  Although  one  may  touch  it  at  any  one  point 
the  kiln  is  so  large  that  about  forty  per  cent,  of  the  heat  is  lost 
in  this  way.  This  type  of  a  kiln  may  be  insulated  rather  easily. 
The  crown  is  not  so  large  as  in  the  periodic  kiln  and  hence  under 
less  pressure.  Being  under  less  pressure  the  crown  would  not  be 
so  liable  to  fall  if  the  kiln  were  more  thoroughly  insulated.  Fur¬ 
thermore,  the  insulation  may  be  graded  to  suit  conditions.  In 


27 


the  type  of  continuous  kiln  where  the  fire  moves,  each  part  must 
be  insulated  to  withstand  the  maximum  temperature,  but  in 
this  type  of  a  kiln  such  a  procedure  is  not  necessary.  The  tem¬ 
perature  at  each  point  is  fixed  and  the  insulation  may  be  adapted 
to  the  condition  found  at  each  part  of  the  kiln.  It  is  expected, 
therefore,  that  a  study  of  heat  insulation  will  result  in  increased 
thermal  efficiency  of  the  tunnel  kiln. 


28 


Other  Types  of  Tunnel  Kilns 

1.  The  Owens  Kiln 

The  tunnel  kiln  designed  by  J.  B.  Owens  is  in  operation  at 
Metuchen,  New  Jersey.  It  is  used  for  the  burning  of  floor  and 
wall  tile,  fire-brick,  etc. 

In  many  respects  this  kiln  is  similar  to  the  Didier-March 
kiln.  It  is  not  a  muffle  kiln,  saggers  being  used  for  the  burning 
of  floor  and  wall  tile.  The  in-coming  air  is  drawn  over  the  hot 
cooling  ware  and  in  that  way  is  preheated,  which  accounts  for 
the  small  amount  of  fuel  necessary  for  its  operation. 


• 

z: 

— r 

£: 

T 

I 

§i 

Fig.  M. 

Outlet  of  Kiln,  Showing  Cars  Loaded  with  Tile  in  Saggers  Just  Drawn  from 
Kiln  Ready  for  Unloading 
(Patent  Pending  on  this  Style  Sagger) 


29 


mmmm 


The  cars  are  introduced  into  the  kiln  intermittently,  the 
intervals  being  gauged  by  the  character  of  the  ware  being  burned. 
The  method  employed  has  already  been  described,  being  essenti¬ 
ally  the  same  as  that  used  in  the  Didier-March  kilns.  The 
mechanism  used  is  shown  in  Figure  N. 


Fig.  N. 

Entrance  to  Kiln,  Showing  Transmission,  End  Door  and  Loaded  Car  Ready 

to  Enter 

(Patent  Pending) 


The  cars  are  lighter  than  the  cars  used  in  the  other  tunnel 
kilns.  No  special  shapes  are  necessary,  standard  fire-brick  being 
used  in  the  top  of  the  car.  The  capacity  of  the  car  is  good,  no 
space  being  lost  as  is  the  case  with  the  car  used  in  the  Dressier 
kiln. 


30 


Oil  is  used  as  the  source  of  fuel  at  Metuchen,  New  Jersey, 
having  been  installed  as  soon*  as  the  price  of  coal  advanced. 

The  insulation  of  the  kiln  is  not  as  thorough  as  that  of  the 
other  kilns.  Because  of  this  there  is  a  marked  difference  in  the 
cost  of  construction,  the  Owens  kiln  being  only  one-half  as  expen¬ 
sive  as  some  of  the  more  complex  and  massive  kilns. 

The  Dressier  Kiln 

The  ware  enters  this  type  of  kiln  at  point  0  (Figure  J).  It 
is  placed  on  cars  which  are  introduced  continuously  by  means 
of  a  hydraulic  ram  located  at  position  F.  As  this  car  progresses 
the  preceding  cars  are  pushed  through  at  the  same  time.  The 
rate  at  which  this  “train”  progresses  depends  entirely  upon  the 
ware  being  fired. 

The  ware  first  becomes  water-smoked  and  then  becomes 
heated  according  to  its  position  in  the  kiln.  As  it  does  become 
heated  the  circulation  of  the  hot  air  in  the  kiln  takes  place  as 
follows  (see  Figure  L) 

The  hot  air  from  the  combustion  chambers  gradually  rises 
through  the  passages,  L,  until  the  top  of  the  kiln  is  reached.  Here 
it  strikes  the  cooler  ware  and  falls,  passing  through  the  ware  R, 
and  the  temporary  air  passage  M.  From  here  the  cooler  air 
passes  through  the  passages  N,  which  are  in  the  car,  over  to  the 
hot  combustion  chamber. 

In  this  way  the  air  moves  in  a  spiral  path,  tending  to  make 
the  cooler  portions  warmer,  and  the  warmer  portions  cooler,  with 
the  resulting  small  temperature  variation  found  in  this  type  of 
kiln  (less  than  one  cone). 

As  the  car  passes  the  hot  zone  an  entirely  different  condition 
occurs.  The  ware  is  now  hotter  than  the  cooling  pipes  which 
take  the  place  of  the  combustion  chambers.  The  air  now  rises 
up  through  the  hot  ware,  strikes  the  cool  roof  of  the  kiln  and 
settles  again  (Figure  K). 


31 


Dressier  Tunnel  Kiln 


A — Gas  Producer 

B — Gas  Duct 
C — Car  Track 

D — Combustion 

Chambers 

O — Mouth  End  of  Kiln 


Fig.  J. 

E — Exhaust  Fan 

P— Tail  End  of  Kiln 
F — Propelling 

Apparatus 
G — Gas  Inlet 

G1 — Air  Inlet 


G2 — Outlet  of  Hot  Flue 
Gases 

H — Pipes  in  Cooling  Zone 
I — Cooling  Zone  Pipe  Out¬ 
lets 

K — Inspecting  Chamber 
S — Insulating  Material 


Cross  Section — Hot  Zone 


Fig.  L. 


D — Combustion  Chambers 
R — Ware  to  be  Fired 


LMN— Air 
S — Insulating  Material 


32 


Cross  Section — Cooling  Zone 


Fig.  K. 

H — Cooling  Pipes 

In  this  way  the  incoming  air  becomes  heated  to  a  high  temp¬ 
erature  and  is  available  for  use  either  in  drying  or  for  the  com¬ 
bustion  of  the  fuel.  It  is  found  that  the  best  results  are  secured 
when  the  heated  air  is  divided,  part  being  used  for  each  of  the 
two  possible  purposes.  Were  the  air  used  entirely  for  combustion 
an  excess  would  be  present  and  the  efficiency  of  the  kiln  would 
be  lowered. 

The  ware  finally  reaches  the  point  P  (Figure  J),  where  it 
is  taken  from  the  kiln  at  a  temperature  so  low  that  glazed  ware 
does  not  dunt. 

This  kiln  is  generally  fired  with  producer  gas  or  natural 
gas,  or  with  both  according  to  the  time  of  the  year.  There  is  no 
reason  why  oil  would  not  be  used  as  well  as  in  the  Owens  kiln. 

The  cars  used  are  shown  in  Figures  L  and  K.  The  ware, 
R,  is  placed  upon  these  cars  in  two  sections  which  are  separated 
by  the  temporary  air  passage  M.  The  air  passages  N  are  perm¬ 
anently  located  in  the  top  of  the  car. 


33 


The  car  moves  on  small  rolls  rather  than  wheels,  these  rolls 
being  cooled  by  air  pipes  which  are  located  in  such  a  way  that 
the  rolls  do  not  rust  or  scale  due  to  heat.  This  method  of  cooling 
the  rolls  gives  satisfaction,  as  does  the  sand  seal  of  the  other  cars. 

The  combustion  chambers  D  serve  as  muffles,  the  heat  being 
taken  from  them  by  the  circulating  air.  Because  of  the  high 
temperature  necessary  to  mature  refractories,  these  muffles  are 
made  of  carborundum,  no  other  material  being  as  capable  of 
withstanding  the  intense  heat  without  deterioration. 

From  10  to  12  per  cent,  of  fuel  is  necessary  for  burning  ware 
to  1250°C.  (cone  6  or  2282°F.). 

Draft  is  maintained  by  fans  and  is  controlled  very  accur¬ 
ately.  The  artificial  draft  makes  it  possible  for  very  accurate 
control  of  the  kiln  atmosphere. 

The  kiln  is  more  expensive  than  the  other  tunnel  kilns  be¬ 
cause  of  its  more  complex  structure.  This  feature  coupled  with 
the  lower  capacity  of  the  cars  are  the  only  bad  features  of  the 
kiln.  They  are  to  be  met  by  the  uniformity  in  temperature  which 
is  secured  and  the  lack  of  flashing.  The  continuous  motion  of 
the  ware  eliminates  any  sudden  increase  in  temperature  of  the 
ware,  and  the  artificial  draft,  as  has  been  said,  results  in  good 
control.  These  latter  features  are  applicable  to  any  kiln. 

Special  arrangements  are  made  for  the  payment  for  such 
kilns. 

INDUSTRIAL  FELLOW 

MELLON  INSTITUTE  OF  INDUSTRIAL  RESEARCH 
UNIVERSITY  OF  PITTSBURGH. 


34 


